test(split-pane): add unit tests for 2/3-pane, cookie-restore, mobile tabs

Added jsdom + @testing-library/react devDeps for component testing.
7 cases: render, divider count, cookie restore, invalid cookie fallback,
mobile tab render/switch, and no-dividers-on-mobile.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -62,6 +62,8 @@
     "@mermaid-js/mermaid-cli": "^11.12.0",
     "@tailwindcss/postcss": "^4",
     "@tailwindcss/typography": "^0.5.19",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
     "@types/bcryptjs": "^2.4.6",
     "@types/node": "^20",
     "@types/pg": "^8.20.0",
@@ -73,6 +75,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.2.4",
     "husky": "^9.1.7",
+    "jsdom": "^29.1.1",
     "lint-staged": "^16.4.0",
     "prisma-erd-generator": "^2.4.2",
     "tailwindcss": "^4",